Newton's Second Law of Motion and Free Fall

8/17/2019 Newton's Second Law of Motion and Free Fall

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Chapter 4:

Newton's 2nd

Law of Motion


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Remember Newton’s 1st Law of Motion?

“Every object continues in a state of

rest or of uniform speed in a straight

line unless acted on by a nonzeronet force.”

- from Newton's Principia Mathematica

Philosophiae Naturalis


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Acceleration ~ net force Acceleration ~ net force

Force causes Acceleration

Acceleration depends on the net force. Acceleration is directly proportional to net force.

To increase the acceleration of an object, youmust increase the net force acting on it.

Acceleration also has the same direction as thenet force that causes it.


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Mass resists Acceleration

The same force applied to:

Twice the mass produces half the acceleration. 3 times the mass, produces 1/3 the acceleration.

• Acceleration is inversely proportional to mass.

Acceleration ~ Acceleration ~ 11massmass


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Newton’s Second Law of Motion

- Isaac Newton was the first to connect the

concepts of force and mass to produce

acceleration.

Acceleration ~ Acceleration ~

massmass

net forcenet force


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Newton’s econd !aw " The !aw of Acceleration

relates acceleration to force and mass.

#The acceleration produced $y a net force on an o$%ectis directly proportional to the net force& is in the same

direction as the net force& and is inversely proportional

to the mass of the o$%ect'”

Newton’s econd !aw " The !aw of Acceleration

relates acceleration to force and mass.

#The acceleration produced $y a net force on an o$%ectis directly proportional to the net force& is in the same

direction as the net force& and is inversely proportional

to the mass of the o$%ect'”


Acceleration ~mass

net force


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n e!uation form:

Example:

f net force acting on object is doubled ⇒

object"s acceleration will be doubled.

#ut if the net force and the mass of object is doubled⇒

object"s acceleration will stay the same.

net force


Accelerationmass


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Mass vs. Weigt

Mass

A measure of the amount of matter or

inertia of a physical object.

ndependent of (ravity .

!reater inertia⇒ greater mass

$easured in )ilo(rams *)(+.

,ei(ht The force on an object due to (ravity .

%cientific unit of force is the Newton *N+.


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- $ass and weight are often used interchan(ea$ly in e&eryday con&ersation.

- #ut mass, howe&er, is different and more

fundamental than weight.

• -or e.ample The weight of an object on the $oon

is one-si(th less than on )arth. #ut the mass of an object is the

same in both locations.

Mass vs. Weigt


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Mass vs. Weigt

* +ilogram weighs * newtons./ newtons to be precise0.

1elationship between +ilograms and pounds: 1 )( 0 ' l$ 2 1 N 2at the )arth"s surface3 1 l$ 0 4'45 N


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Friction

4riction occurs when surfaces slide o&er oneanother.

t reduces the net force and in turn also theacceleration.

4riction always occurs opposite the direction ofmotion.


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"e Force of Friction

• depends on the )inds of

material and how much

they are pressed together.

• is due to tiny surface

$umps and to the

#stic)iness6 of the atoms

on a material"s surface.

)(ample: 4riction between a crate on a smooth wooden

floor is less than that on a rough floor.


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"e Force of Friction

• "wo "#$es of Friction%

TATIC friction +eeps the object from mo&ing

to begin with.

LIDING friction goes against the direction

of motion while an object is mo&ing.

• 5epending on the surface, static friction is

generally (reater than sliding friction.


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Notes about Friction

• The force of friction does N&" depend on the

speed .

• 4riction does N6T depend on the area of contact .

• Also, friction is not restricted to solids sliding o&er

one another, but to li!uids and gases a+a 'fluids'0

as well.


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PHYS 1305: Intro to PhysicsI

Chapter 4:

FREE


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Free Fall

The (reater the mass of the

object7 the (reater its force of attraction

toward the )arth. the smaller its tendency to mo&e

i.e., the greater its inertia.

%o, the acceleration is the same because the ratio of the force of

(ravity to the mass of the object

is the %A$).


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Free Fall


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Non'Free Fall

8hen an object falls downward

through the air it e(periences

- force of gra&ity pulling it

downward.

- air drag force acting upward.

m( 78 Acceleration 0

m


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Non'Free Fall

,hen acceleration of fall is less than ( 9a : (;&

non7free fall

•occurs when air resistance is non7 ne(li(i$le or si(nificant .

• depends on two things:

*. speed 9. frontal surface area'


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Non'Free Fall

8hen the object is mo&ing fast enough thatforce of gra&ity e


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Non'Free Fall

Terminal speed occurs when acceleration

terminates when air resistancee!uals weight and net force is

;ero0.

Terminal velocity same as terminal speed, with

direction specified.


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Non'Free Fall

•less

force weight0 for air

resistance to o&ercome.•

less

time to accelerate.•

1

st

to reach terminal

&elocity.

•slower

terminal &elocity.•last

to reach the ground.

•MORE

force weight0 for air

resistance to o&ercome.•

MORE

time to accelerate.•

last

to reach terminal

&elocity.•

FA TER

terminal &elocity.•

1

st

to reach the ground.


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Free Fall vs. Non'Free Fall

Without air in vacuum! With air no vacuum!

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Newton's Second Law of Motion and Free Fall